One for @pkrboo Interesting article, I've read about this before but this article is a good one. https://www.zwerfcat.nl/en/lithium-hybrid.html
Interesting way of solving the problem of as he says apparent incompatibility. It would also work well for starter battery applications. Too much current the voltage drops and the BMS disconnects leaving the lead acid battery to do its work.. In the end though using supercapacitors would also work for starting - essentially one of the better quality supercap starter packs in parallel with the lithium battery. Charge it off the leisure battery, then try a start. Wait a bit and it recharges for another go.. Still there is the problem that with some smart lead acid chargers ... if you are draining any current off the lead acid batteries while trying to charge them they can never reach 100 percent as the charger doesnt understand a battery that drops and bumps up in voltage as you turn on and off the load. So e.g. with a 5 amp fridge attached to the leisure battery the 10 amp lead acid battery charger never gets the battery to 100%.
The suggestion is more about simple addition of Leisure Lithium battery that by itself solves the tail charge of the Lead battery. Mostly you'd use the Lithium but that's another win as it charges fast. I'm almost game for the boat.
Its interesting because I can imagine a system where the leisure battery is lithium, and it sits in parallel with the starter battery - with some kind of VSR style disconnect between the two at 12.1 volts or whatever, when there is still enough juice to start the engine, and the lithium BMS has already disconnected. Which is basically a differently tuned BMS on the starter battery to leisure link.
Now there's an idea. My biggest problem may be keeping the lithium batteries warm enough in the winter. I want them near the charging for obvious reasons. I have a 120A charger that will run on my engine powered Travel Power at surprisingly low RPM, maybe 1500, It's a dandy thing. It would be even better if I could parallel charge with the 100A leisure alternator and whack 220A+ into them for and hour while the engine also heats a tank of water. There's even a third 60A alternator to charge the starter battery - I'll probably rip the end off the crank! That's if it would even all work together like that, I haven't begun to think about it really, I watch others though, I'm waiting while one guy sweats over his low and high voltage cut-outs for £3K of batteries. He's not a particularly well off chap, he can't afford to mess up and can't afford ready made kit to control it all. D.c. switching, heavy duty gear. It's a shame my father has lost his marbles, he was high power D.C. engineering when I was a toddler on some train that went up a mountain in South Africa. Later it was anti-lock brake control design but he was never encouraging when I suggested he help me anti-lock my camper brakes.
Another chap I chat with plans to charge his at 350A, he's a brain box and busy designing his own battery management to use all his alternators IIRC, he has the exact same engine, alternators and 240 travel power which could turn out to be handy but a bigger charger/inverter. I thought mine was quite hefty.
The lead acid battery provides some heat to start the water cooled lithium batteries up , or runs a heat pump. In the summer you run water through a hull cooling plate for the lithium batteries to cool them down.
